320 DISEASE IN WILD MAMMALS AND BIRDS 
preserved a long while in any liquid it may be found con- 
tracted or broken. Large vacuolated or vesicular cells 
are characteristic of the human parathyroid ; this type is 
by no means so prominent in the lower animals, and in the 
few examples studied closely the arrangement is less 
definitely glandular than in man. 
The avian thyroid is distinct from the mammalian in 
the delicacy of its fibrous framework and the flatter 
character of its epithelia. Acini are usually of more 
uniform size. Capsular vessels are prominent but 
internal vascularity is less in birds than in mammals and 
their goitres are not solid. 
Physiology. 
The physiological value of the thyroid-parathyroid 
complex has been the subject of extensive study and 
voluminous literature without exhaustion of the possi- 
bilities, but with the result that we are possessed of 
knowledge explaining certain phases of abnormality, even 
if the normal fmictions be not unexceptionally demon- 
strated. The accepted alterations of functions are 
hypothyroidism — inadequate physiology, and hyperthy- 
roidism — excessive activity. Absence or atrophy of the 
thyroid bodies is usual in hypothyroidism, while enlarge- 
ments, collectively called goitre, commonly accompany 
excessive function. Exemplifying the former, cretinism 
is the result of failure of normal function and develop- 
ment during fetal life while myxedema is the expression 
of the disappearance of thyroid secretion after it has once 
been operative; the latter may occur in infancy after 
nursing has ceased, or at any time that the thyroid may 
atrophy, during some of the forms of goitre for example. 
Hyperthyroidism may express itself, with or without 
visible enlargement of the thyroid body, in nervousness, 
gastrointestinal disturbances, tachycardia, loss of weight 
and exophthalmos. The first group, which might be called 
athyroidism, is often associated with alterations in the 
